A boring mill is a machine used to bore holes in steel. Most commonly used to enlarge cylinder bores in automotive engine blocks, the boring mill is a very useful tool in any machine shop. There are both horizontal boring mills as well as vertical types. Typically, the vertical boring mill will be used to over-bore an engine block's cylinders, while the horizontal boring mill is used to re-size the engine block's main bearing saddles in a process known as line honing.

When an automotive engine has seen too many miles and is running poorly, it is often rebuilt. In the process of rebuilding an engine, it is often necessary to bore the cylinders over size to correct out-of-round or tapered conditions. The boring mill is used to open the cylinder up to a uniform bore size from the top of the cylinder to the bottom. By removing only the minimal amount of material from the bore to create an even cylinder wall, the structural integrity and strength of the engine block is not diminished.

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In an engine with a multitude of miles or simply a poor maintenance history, it often becomes necessary to re-size the main bearing saddles. By removing material from the saddle mating surface, the area that supports the main bearings becomes out-of-round and too small for the crankshaft to be installed in the block. In severe cases, the horizontal boring mill is used to bore the main saddles to the proper size again. As in any boring process, the task is finished by honing with a fine stone.

Boring is not limited to only cylinder walls and main saddles. Many areas of the engine block are treated to some time spent on the boring mill. This is especially true in high-performance applications. Lifter bosses, cam-bearing journals as well as piston pin lands are all sized on a boring mill. With a skillful operator, a boring machine can bring life back to an engine that otherwise would be given up for scrap iron.

In the case of a broken or cracked cylinder wall, the operator can actually bore the entire cylinder wall out of the engine block and install a cast iron sleeve. Once fitted, the sleeve is honed to the proper size and a new piston assembly is installed into the reconditioned bore. Properly installed, a sleeved engine block is as durable as a factory bore, and in many cases, it proves to be more durable.

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